bad variable escape analysis when TypeFamilies are enabled

The following minimal example causes a type error in 7.3.20110927:

{-# LANGUAGE TypeFamilies #-}

class Foo a where
	foo :: a -> a

instance (Foo a, Foo b) => Foo (a, b) where
	foo = foo' ()

foo' es = const id (unitId es)

unitId :: () -> ()
unitId = id

Specifically, the error given is:

test.hs:6:10:
    Couldn't match type `a0' with `(a, b)'
      because type variables `a', `b' would escape their scope
    These (rigid, skolem) type variables are bound by
      the instance declaration
    The following variables have types that mention a0
      foo' :: () -> a0 -> a0 (bound at test.hs:9:1)
    In the instance declaration for `Foo (a, b)'

This code compiles successfully as recently as 7.3.20110726 (though I haven't tested any GHC versions in between these two).

Trac metadata

Trac field	Value
Version	7.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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